By Jeffry Ceruti, P.E.
The near-record snowfall in Massachusetts this year, and the lack of melting sprees between
storms, created a deep and dense snowpack on many roofs this winter. In addition to weight
concerns, this snowpack provided ideal conditions for formation of icicles and ice dams on many
sloped roofs, causing damaging water leakage in many buildings.
Ice dams form by heat loss through the roof— the heat loss causes snow on the roof to melt,
and this meltwater flows to the cold roof eave overhang and re-freezes. Gutters contribute to ice
dams because the meltwater collects and freezes, filling the gutter with ice. As the ice formation
grows, the continued melting that occurs upslope becomes trapped causing water to back up
under the roofing and eventually leak into the building.
Removing snow from the roof can be effective at controlling leaks, but is an intense chore that
must be done after each major storm. This work can also cause damage to the roofing making
the leaks worse. Below are several other ways to deal with ice dams— some proactive and
many reactive. The best solutions treat the cause of the problem and not the symptoms.
Snow Melt Systems: Snow melt systems, consisting of electrically heated cables, are attached
to the roof to melt portions of the snow/ice, allowing the meltwater above to flow off the roof.
While these systems can be effective, they are prone to damage from snow and ice that creeps
down the roof. Sliding snow can tear the cables off and cause shorts that make the system
ineffective when it is needed most— this can happen after the system clears a section of snow
at the eave, allowing snow upslope to slide down. If used, the heating cables should extend to
a point where ice build-up can occur without causing other problems— usually on grade away
from walkways or driveways. Expect that some maintenance of the system will be needed each
spring.
Membrane Ice Dam Protection: Membrane ice dam protection, installed below the primary
roofing, is often an effective way to control leaks that are associated with ice dams. These
membranes are typically made of rubberized asphalt and polyethylene, and are self-adhering
to the roof deck/sheathing. Most modern roof installations in New England include ice dam
protection at roof eaves and valleys; installing this on an existing roof requires removing and
replacing the roofing near the eave. While the membrane does not prevent formation of ice
dams, if installed correctly it can provide a watertight assembly that keeps the backed-up water
from entering the building. A few issues to remember with regard to ice dam protection include:
– At roof eaves, a rule of thumb is to extend the membrane 3 ft upslope of the exterior
wall. The membrane should also turn up to the same height at any dormers or
side walls that occur within this area.
– Roof orientation that concentrates runoff to small areas of the roof are particularly
prone to large ice build-ups; the membrane must be extended up higher in
these areas. Valleys tend to collect large volumes of snow/ice and should also
be protected with membrane underlayment.
– Large and repeated ice dams may damage the primary roofing— ice expansion
can loosen roofing and flashing attachments, and open fastener holes in the
membrane underlayment.
Metal Roofs and No Gutters: In northern New England, many sloped roofs are covered with
metal roofing or have small sections of metal installed at the eaves, and many roofs do not
have gutters. Metal roofs allow snow to slide off the roof quickly before ice dams can form, thus
eliminating the problem. The lack of gutters also prevents the snow/ice from accumulating at
the roof edge. A safe and clear area must be present at grade for sliding snow to accumulate. If
gutters are removed, site drainage may also need to be considered to control roof runoff.
Vent beneath the Roof Deck: The most effective way to control ice dams is to vent beneath
the roof deck to control heat loss through the roof. With sufficient ventilation, the roof
surface stays close to outdoor temperatures thereby limiting the melting of snow during cold
temperatures. Venting is typically achieved by a “cold attic” design, or including a vent space
within the roofing system. Soffit vents and ridge vents are needed to provide an effective
system— stack pressure allows cold air to enter at the soffit and flow out of the ridge. The vent
space must be unobstructed, and sized properly for the length of the rafters. Key issues with a
vented roof assembly include:
– Thermal insulation for the building must be below the vented space. For cold attics,
insulation is typically in the attic floor; for cathedral ceilings, insulation is within
the rafters with a vented channel between the insulation and roof deck.
– A continuous air barrier is important to control heated interior air from flowing up into
the vent space or vented attic, causing snow to melt. Painted gypsum finishes
can be effective for this purpose, but interruptions (such as recessed lighting)
must be well sealed to prevent air bypass.
– Heating system elements (units, ductwork) should not be placed in a cold attic, as the
heat given off will negate the effects of venting. If this is unavoidable, then all
ducts should be sealed airtight and well insulated, and the units themselves
should be isolated somehow to avoid contributing heat to the cold attic.
– Dormers and other changes in plane can interrupt ventilation by eliminating portions
of the eave or ridge vent— this must be accounted for in the ventilation
calculations.
– Complicated roof geometry, where runoff from large roof areas is concentrated to small
locations, can create ice dams regardless of the amount of ventilation. Multiple
dormers aligned along the eave restrict snow and ice between them, and can
cause significant ice dams.
Super-Insulated Roof Assemblies: Super-insulated roof assemblies, typically using closed-
cell spray-applied urethane foam insulation, can help to reduce the frequency and magnitude of
ice dam formation without the use of ventilation by slowing heat loss significantly. Be aware that
these products have low vapor permeability— if roof leaks develop, the polyurethane foam may
not allow leakage water to dry, promoting deterioration of the roof deck over time.
Choosing the best way to combat ice dams may vary for each building and owner, based on the
building design, the tolerance for leaks, and the financial resources available.
About the Author:
Jeffry Ceruti, P.E., is a senior principal at national engineering firm Simpson Gumpertz & Heger
Inc. (SGH). Jeffry specializes in the design, investigation, and rehabilitation of building envelope
assemblies. His experience includes steep- and low-sloped roofing systems, plaza and below-
grade waterproofing, various exterior wall systems, windows and curtain walls, and other
building envelope systems.
